Tiffany is 2 times as old as Christopher. 21 years ago, Tiffany was 5 times as old as Christopher. How old is Christopher now?
Answer: We can use the given information to write down two equations that describe the ages of Tiffany and Christopher. Let Tiffany's current age be $t$ and Christopher's current age be $c$ The information in the first sentence can be expressed in the following equation: $t = 2c$ 21 years ago, Tiffany was $t - 21$ years old, and Christopher was $c - 21$ years old. The information in the second sentence can be expressed in the following equation: $t - 21 = 5(c - 21)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $c$ , it might be easiest to use our first equation for $t$ and substitute it into our second equation. Our first equation is: $t = 2c$ . Substituting this into our second equation, we get: $2c$ $-$ $21 = 5(c - 21)$ which combines the information about $c$ from both of our original equations. Simplifying the right side of this equation, we get: $2 c - 21 = 5 c - 105$ Solving for $c$ , we get: $3 c = 84.$ $c = 28$.